1. Field of the Invention
The disclosed invention relates generally to the problem of maintaining an ink jet print head in a desired operating condition. More specifically, it relates to the problem of purging and cleaning a print head which utilizes a solid ink that melts into liquid form when heated.
2. Description of the Prior Art
U.S. Pat. No. 4,658,274, issued to De Young, Apr. 14, 1987, describes a hot melt type of ink jet printer in which a print head is heated during operation in order to maintain a hot melt ink in a liquid phase. The liquefied state has to be maintained during periods when the ink is to be hydro-mechanically ejected as a plurality of melted droplets from tiny orifices.
Known solid inks tend to degrade over time when held at liquid temperatures. It consistent print quality is desired, an unused volume of ink which has been held in a heated state for too long should be replaced with a fresh volume ink.
FIG. 1 is a partial top view of a previous printer 10 having a frame 10a and including a purging system such as proposed in U.S. Pat. No. 4,658,274 to DeYoung. The printer 10 includes a movable print head 11 which is reciprocably supported on a pair of guide rails 12. The guide rails are fastened to the frame 10a. The head 11 is controlled to move back and forth along the rails from a printing area 14 in the printer to a head maintenance area 16. A rigid V-tipped tube 22 is provided within the maintenance area 16 for engagement with a vent hole 11a of the print head 11. An air pump 20 supplies pressurized air through a flexible line 21 to the V-tipped rigid tube 22. The V-tipped tube 22 is reciprocally disposed in a bushing 24 that is fastened to the frame 10a as shown.
When the print head 11 is to be purged, the print head is moved to bring the vent hole 11a of the print head into engagement with the V-tipped tube 22. The air pump 20 is then activated to pump air through the vent hole 11a into the interior of the print head. The level of air pressure at the vent hole 11a, relative to the ambient air pressure, controls the rate at which melted ink 15b within the print head moves up a capillary tube 17 to be expelled from one or more orifices 18 (only one shown) passing through a printing face 11d of the print head 11. The melted ink 15b is usually held within the print head in a reservoir 11b. The temperature of the reservoir is controlled by a heater 11c. A solid ink cartridge 15, containing pellets 15a of solid ink, supplies fresh ink material to the reservoir 11b whenever the liquid ink 15b in the reservoir drops below a predetermined level.
During a normal printing mode, the vent hole 11a is left open so that the air pressure inside the print head 11 is approximately the same as the air pressure outside the print head. A plurality of piezoelectric transducers 19 (one shown) are used to fire ink droplets 15c from the plural orifices 18 (one shown) the print head to a spaced away page of paper 19 held on a platen 10c. The platen 10c is fastened elsewhere to the printer frame 10a. The paper 19 is preferably spaced by a precise distance CC away from the printing face 11d of the print head to assure good print quality.
When ink jet printers of the type shown in FIG. 1 are mass produced, a phenomenon known as "tolerance build-up" can occur. A substantial misalignment can develop between the V-tipped tube 22 and the vent hole 11a even though the bushing 24 is positioned within a prespecified tolerance relative to the printer frame 10a, the carriage rails 12 are also attached within a set tolerance relative to the frame, and the print head 11 is held within a specified tolerance relative to the carriage rails 12. Positional deviations, DD, can add up between the respective axes, AA, BB, of the tube and vent hole such that the V-tipped tube 22 will not properly align with the vent hole 11a of the print head. The pressure level during a purge operation becomes uncertain when this happens.